Composite laminar structures are strong and light-weight. Their use is well known and they are frequently used in automotive, aerospace, sporting goods and marine applications.
Typically composite materials are manufactured by stacking layers of a fibrous reinforcement material which is preimpregnated with a curable resin material (prepreg). The resin material is then cured by heating the stack whilst it is being compressed. This causes the resin to flow to consolidate the fibrous stack, and then to subsequently cure. This results in an integral laminar composite structure.
Composite materials can also be formed by arranging layers of dry fibrous material into a mould and then infusing with a curable resin. The resin wets out the fibres of the dry material before being cured. This process is known as resin transfer moulding (RTM).
Both methods result in a composite material with a laminar structure having a series of layers of impregnated fibrous reinforcement. Between these layers is a resin rich layer distinguished by an absence of reinforcement fibres known as the interleaf or interlayer.
Thermoset resins, and in particular epoxy resins, can be brittle due to their cross-linked interpolymer networks. The highly cross-linked thermoset resin matrices that are used for more high strength applications such as aerospace structures can be exceptionally brittle. There exists a need improve their toughness in order to be suited for such applications. This is normally achieved with the addition of tougheners to the resin matrix. Typically thermoset matrices are toughened by the addition of a second polymer such as an elastomeric or thermoplastic additive. Phase separation of the second polymer from the matrix polymer introduces toughening mechanisms that improve fracture toughness. Epoxy resins modified with the addition of rubbers are one such example of this. The addition of a rubber to a matrix has however been found to reduce matrix modulus, yield strength and glass transition temperature. This approach to toughening is therefore unsuitable for many applications where high strength is required. The addition of certain thermoplastics can provide similar toughening but with a minimal reduction of the mechanical properties that are otherwise degraded by rubber. Soluble thermoplastics can be added to the matrix which phase separate during cure, or insoluble particles can be added which remain in a separate phase throughout processing. It is believed that the separate thermoplastic phase in the cured matrix acts to toughen the matrix through a variety of proposed mechanisms, including crack path deflection, particle yielding, shear banding, crack bridging, crack pinning and micro cracking.
The addition of any particulate additive to a matrix e.g. a toughener or a conductive particle, has the undesirable effect of increasing the viscosity of the matrix. A high matrix viscosity presents numerous processing difficulties. It is therefore necessary for a matrix comprising particulate additives to be heated to reduce its viscosity during various stages of processing.
The high temperatures required to process the resin composition can often exceed the melting point of the additives. Where the additives comprise thermoplastic toughening particles, the result is an undesirable melting of the particles. The particles can then agglomerate or deform, impairing their ability to toughen the host matrix. The same effect may also occur during exothermic cure of the matrix. One solution to this is to use thermoplastic particles with a higher melting point but these do not tend to provide optimal toughening. Alternatively lower temperatures and lower process rates can be used, but this results in a decrease of line speed and rate of production.
Electrical conductivity is a desirable characteristic of a composite material used in aerospace and wind energy applications. It is particularly important for a composite material to possess a suitable level of conductivity in applications that may be vulnerable to lightning strike. The addition of thermoplastic tougheners to composite materials also results in an undesirable decrease of electrical conductivity. A thermoplastic toughener that retains matrix conductivity is therefore highly desirable.
Particular additives are not typically used in composite materials made by RTM because their flow is impeded by the reinforcement fibres and they become unevenly distributed throughout the cured composite. Instead, a common way of toughening infused composite materials is to place a thermoplastic non-woven fabric in the form of a veil between the fibre layers in the mould before the resin is infused. During cure the heat can cause the veil to melt and lose its structure. This can result in an undesirable reduction of toughening because the veil melts into irregular shapes and flows into the fibrous reinforcement tows. These irregular shapes can also be mistaken for defects during NDT inspection, sometimes leading to accidental scrapping of a composite part. The term “veil” is used to describe a thin lightweight porous web or fibrous reinforcement having a weight in the range of from 0.5 to 30 g/m2 (gsm), preferably from 1 to 25 gsm, and more preferably from 1 to 10 gsm, and even more preferably from 1 to 8 gsm and/or or combinations of the aforesaid weight ranges which allows liquid resin to pass therethrough, typically it is fibrous and also it is preferably derived from a polyamide.
The present invention aims to overcome the above described problems and/or to provide improvements generally.